THE AVONIAN OF THE AVON GORGE 
97 
initiated at the surface of the mass, and therefore, practically 
at sea-level. 
The conclusions thus arrived at, as to shallow water conditions 
during Syringothyris-timQ, are confirmed by an examination of 
the strata of the same age in other parts of the South-Western 
Province : — 
At Weston-super-Mare, the rocks of this age contain inter- 
bedded lava and ash, of such a nature as to indicate shallow 
water and proximity to land. Farther west at Pendine, north 
of Tenby, there is a gap in the succession and the Syringothyris- 
Zone is absent, indicating that, locally, the sea floor emerged 
and became land during this period. 
Zaphrentis-Zone (Z). 
All the beds in the Black Rock Quarry below the 
Plate III. j 
‘ ^ummosu-dolomites ’ belong to the Zaphrentis -Zone 
and are of essentially the same rock-type, namely a highly- 
fossiliferous, encrinital limestone. The divisions are consequently 
entirely palaeontological. 
The uppermost part constitutes Horizon y and the steep slope, 
just below the ‘ Cave,’ may be taken as the dividing plane between 
the upper and lower subzones of the Zaphrentis-Zone (Zi and Z^.) 
The basal beds of the ‘ Black Rock ’ massif constitute Horizon 
yS and are exposed in the small quarry north of the Black Rock 
Quarry. (This small quarry, here named Press’ Quarry, is now 
closed and forms part of a private estate.) 
The whole of the Zaphrentis-Zone, including Horizon y at the 
top and Horizon ^ at the base, may be considered to have been 
deposited under standard conditions. 
Gleistopora-Zone (K). 
Upper subzone (K2). 
Plate I railroad in front of Press’ 
Quarry, we again resume our walk northward. The 
slope on the right hand side of the line is almost completely 
